A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium mounted in various kinds of magnetic recording devices. More particularly, the invention relates to a perpendicular magnetic recording medium mounted in a hard disk drive (HDD) used as an external memory device of a computer, AV instrument, or the like.
B. Description of the Related Art
As a result of a sharp rise in demand for an increased HDD recording density, a magnetic recording method is shifting from a heretofore known longitudinal magnetic recording method to a perpendicular magnetic recording method, which is advantageous in increasing density. A perpendicular magnetic recording medium (hereafter also abbreviated to being called a perpendicular medium) used in a perpendicular magnetic recording is mainly configured of a magnetic recording layer of a hard magnetic material, an underlayer for orienting the recording magnetization of the magnetic recording layer in a perpendicular direction, a protective layer that protects the surface of the magnetic recording layer, and a back layer of a soft magnetic material that fulfills a role of concentrating magnetic flux emitted by a magnetic head used in a recording onto the recording layer.
As a detailed structure of the magnetic recording layer, a structure called a granular structure, wherein ferromagnetic crystal grains are surrounded and magnetically divided by a non-magnetic grain boundary component, is generally used. In the following description, the ferromagnetic crystal grains may be abbreviated to being called ferromagnetic grains. CoPtCr—SiO2, or the like, wherein the ferromagnetic grains are formed from Co, Pt, and Cr and the non-magnetic grain boundary component is the oxide SiO2, is proposed as a specific example of the granular structure (for example, refer to JP-A-2003-178412).
A characteristic primarily required of a magnetic recording medium is the size of the recording density, meaning how many signals can be written, and in order to realize a high recording density, a miniaturization of the ferromagnetic grains of the magnetic recording layer, or a reduction of magnetic interaction between the ferromagnetic grains, is effective. However, as a depreciation of thermal stability due to a so-called thermal fluctuation occurs when promoting a miniaturization of the ferromagnetic grains, it is necessary to increase a perpendicular magnetic anisotropy energy Ku held by the ferromagnetic grains. It is reported that, with a perpendicular medium, CoPtCr—SiO2, wherein an oxide is added, is superior in the two characteristics of reducing interaction between grains and high Ku in comparison with CoPtCrB, which is a material used in a heretofore known longitudinal medium (for example, refer to T. Oikawa et al., IEEE Transaction on Magnetics (U.S.A.), 2002, volume 38, number 5, pages 1,976 to 1,978). Because of this, it can be supposed that a hard magnetic material with an oxide or nitride as a non-magnetic grain boundary component is more advantageous in increasing the recording density of a perpendicular medium.
Meanwhile, with regard to the strength of a magnetic field generated by the magnetic head that carries out a recording onto a magnetic recording medium in an HDD, it is known that the necessary magnetic field strength is proportional to Ku. Consequently, when Ku is increased, it is also necessary to increase the strength of the magnetic head magnetic field. However, as there is a limit to magnetic field strength, a problem occurs when increasing Ku in that a saturation recording wherein the magnetization of the magnetic recording layer is all oriented in one desired direction becomes difficult. Also, when promoting a miniaturization of the ferromagnetic grain size, a demagnetizing field acting on the ferromagnetic grains is reduced, and this also leads to an increase in a magnetization switching field. That is, there is a trade-off relationship, in that a miniaturization of the ferromagnetic grains and an increase of Ku with a view to increasing the recording density causes a depreciation in the writing performance of the magnetic recording medium. Because of the heretofore described kind of background, there is a demand for a method whereby the signal quality and stability of a magnetic recording medium are improved, while maintaining the writing performance.
In response to this problem, a method is proposed whereby, by making the magnetic recording layer a multilayer structure of two or more magnetic layers and a layer controlling coupling force inserted between the magnetic layers, and weakening the exchange coupling force between the magnetic layers, the switching field is reduced with barely any depreciation of thermal stability. This kind of medium is called an exchange coupling control medium. Although the exchange coupling force is infinite when directly stacking the two magnetic layers, it is possible to control the exchange coupling force by inserting an exchange coupling control layer between the two magnetic layers. When weakening the exchange coupling force with an exchange coupling control layer, the switching field reaches an extremely small value at a certain optimum exchange coupling force. When the exchange coupling force is further weakened, the result obtained is that the switching field increases again when the exchange coupling force approaches zero (for example, refer to J. Magn. Soc. Jpn., 2007, volume 31, page 178). This occurs because each of the two magnetic layers carries out a differing magnetization switching (that is, an incoherent magnetization switching), while also maintaining a weak exchange coupling. As a developed form of this, there is proposed a structure having a total of three magnetic layers, wherein the magnetic layer above the exchange coupling control layer is divided into two, and wherein Ku gradually increases from the upper portion (for example, refer to JP-A-2008-287853).
In the proposed exchange coupling control medium, a layer with a low Ku is disposed in a portion nearest to the magnetic head. As a result of diligent study by the inventor, it is found that an exchange coupling control medium with this kind of structure is disadvantageous in narrowing the track pitch. It is ideal that the recording field of the magnetic head is such that a strong magnetic field is generated in only a down-track direction. However, in actual practice, a weak recording field is also generated in a cross-track direction. Because of this, the cross-track direction component of the recording field affects an adjacent track into which a signal has already been written, and the strength of the signal written is reduced.
As heretofore described, it is found that there is a limit to the narrowing of the track pitch in an exchange coupling control medium thought to be effective as a perpendicular medium providing a good writing performance, signal quality, and stability.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.